1. The Field of the Invention
The present invention relates generally to x-ray devices. More particularly, embodiments of the present invention relate to an x-ray tube cooling system which includes features that serve to permit monitoring various coolant flow parameters, and thereby facilitate safe and reliable operation of the x-ray device.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when free electrons are generated, accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure. Disposed within the evacuated enclosure is an electron source, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then imposed between the anode and the cathode, thereby causing the emitted electrons to rapidly accelerate towards a target surface positioned on the anode. The anode may be a stationary type anode, as is often employed in the context of analytical x-ray tubes, or a rotating type as is commonly employed in the context of diagnostic x-ray devices used in medical applications. During operation of an x-ray tube, the electrons in the beam strike the target surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic or xe2x80x9cZxe2x80x9d number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient""s body, or material sample. As is well known, the x-rays can be used for therapeutic treatment, for x-ray medical diagnostic examination, or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the target anode. As a result, the target anode typically experiences extremely high operating temperatures. At least some of the heat generated in the target anode is absorbed by other structures and components of the x-ray device as well.
A percentage of the electrons that strike the target surface do not generate x-rays, and instead simply rebound from the surface and then impact another xe2x80x9cnon-targetxe2x80x9d surfaces within the x-ray tube evacuated enclosure. These are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated. This heat can ultimately damage the x-ray tube, and shorten its operational life. In particular, the heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. In some instances, the resulting high temperatures can even melt portions of the x-ray tube, such as lead shielding disposed on the evacuated enclosure. Such conditions can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
In view of the significant dangers posed by excessive heat levels in x-ray tubes and devices, various types of cooling systems have been devised to aid in the removal of heat from x-ray devices. For example, many conventional x-ray tube systems utilize some type of liquid cooling arrangement wherein a flow of coolant is generated and directed into contact with various surfaces and components of the x-ray tube so as to remove some of the heat generated there. The heated coolant is typically returned to an external cooling unit which removes heat from the coolant and then returns the coolant to the x-ray device. As discussed below, the configuration of the cooling system may vary somewhat depending on the type of x-ray device with which it is employed.
In the case of stationary anode type x-ray tubes, for example, the liquid coolant is typically injected, by way of a coolant injection nozzle, into a passage defined by the anode. The coolant absorbs heat from the anode and then exits the passage before returning to the external cooling unit.
The configuration of the cooling system is somewhat different in the context of typical rotating anode type x-ray tubes. In particular, many rotating anode x-ray tubes contain structures through which, or over which, a flow of coolant is directed. The coolant absorbs heat as it contacts these structures, and then ultimately returns to the external cooling unit.
It is well known that the ability of a coolant to remove heat is at least partially a function of the flow rate of that coolant. In particular, where two coolant streams are substantially equivalent in all other regards, a coolant stream characterized by a relatively higher flow rate will generally remove heat at a relatively higher rate than a coolant stream having a relatively lower flow rate.
Generally, the coolant flow rate in an x-ray tube cooling system is a function of the amount of heat produced by the x-ray device. Because the failure to maintain an adequate coolant flow rate may result in damage to the x-ray device, x-ray cooling systems using a liquid coolant are typically designed to ensure that a certain minimum of coolant flow rate is maintained. Various types of instrumentation and control systems have been devised and employed in conjunction with liquid cooling systems in attempt to ensure maintenance and/or verification of a minimum acceptable coolant flow rate. As discussed in detail below however, known devices and systems suffer from a variety of shortcomings.
In one known type of cooling system, a direct flow measuring device such as a turbine meter, plunger, or rotameter is included xe2x80x9cin-linexe2x80x9d in the coolant circuit. That is, the coolant must pass through the direct flow measuring device in order for the device to be effective in measuring the coolant flow rate. Typically, such direct flow measuring devices include an electrical switch or the like arranged so that upon achievement of a desired coolant flow rate through the device, contacts on the electrical switch close and complete a circuit. Generally, the circuit includes some type of visual indicator or the like to show that at least the minimally acceptable coolant flow rate has been established.
While direct flow measuring devices are generally effective in indicating coolant flow rates, they nevertheless suffer from some significant shortcomings. One such shortcoming relates to the fluid system energy losses imposed by such devices.
As is well known, the energy of a fluid system is often referred to as the xe2x80x9csystem headxe2x80x9d and includes the energy represented by the velocity and pressure of the fluid in the system. In general, it is desirable to minimize losses in the energy of the system, or xe2x80x9chead loss,xe2x80x9d which would tend to compromise performance of the fluid system. As discussed below however, some head loss is unavoidable.
In particular, the system head is affected by a variety of factors. For example, friction between the fluid and the piping through which it passes tends to reduce the velocity of the fluid, and thus, the overall energy of the system. Further, by virtue of their geometry and other characteristics, the devices and components in the fluid system tend to resist flow of fluid therethrough. This resistance to fluid flow is often described in terms of the xe2x80x9cpressure dropxe2x80x9d (head loss) imposed by that device or component on the fluid. Thus, the devices and components of the system tend to reduce the overall system energy by imposing a head loss, or decrease in pressure, on the system fluid.
Because known direct flow measuring devices are generally characterized by relatively large pressure drops, they tend to undesirably reduce the overall energy of the fluid system and thereby compromise coolant flow and cooling system performance.
Another problem associated with many types of direct flow measuring devices relates to the mechanism by which such devices perform the flow sensing function. In particular, such mechanisms are relatively sensitive and accordingly must be kept free of contaminants and foreign matter so as to preclude any malfunction of the flow measuring device. Because of their sensitivity, such devices typically employ some type of filter which serves to screen out any contaminants and foreign matter that could impair the operation of the device. Although such filters are generally successful in this regard, their use implicates various undesirable consequences.
In particular, the addition of the filter in-line in the cooling system further increases the system head loss and thus compromises the overall performance of the cooling system, as discussed above. Furthermore, the filter represents a cost burden in that it must be incorporated into the x-ray tube, thereby increasing the price of the x-ray tube device.
Finally, as suggested earlier, the filter must be integrated into the cooling system in such a way that it can be readily installed. This functionality is typically achieved by way of pipe fittings, flanges or other removably attachable fluid connections. However, each of these fluid connections represents a point in the cooling system where a leak could occur. By necessitating the use of additional fluid connections in the system, these cooling system filters thus increase the likelihood of leaks and other system performance problems.
While direct flow measuring devices permit, as their name suggests, direct measurement of the rate of fluid flow through the device, various other sensors and cooling system configurations have been employed to sense other flow parameters, such as pressure, which can then be used as a basis for deriving the associated flow rate.
In one such configuration, a differential pressure (xe2x80x9cDPxe2x80x9d) switch is connected across the coolant heat exchanger of the x-ray device and the DP switch is adjusted so that at a pre-determined minimum flow rate, the static pressure drop across the coolant heat exchanger is sufficient to complete a circuit in the DP switch, indicating that at least the minimum flow rate has been achieved. Because the flow rate is known, or can readily be determined, for a given pressure differential, the DP switch, indirectly, facilitates verification that the coolant flow rate is at least at the minimum acceptable level. In the event the pressure differential falls to a point which corresponds to a coolant flow rate lower than the minimal acceptable coolant flow rate, the circuit in the DP switch is opened, indicating an inadequate coolant flow rate.
While the DP switch avoids some of the problems inherent in in-line type flow sensing components such as turbine meter or plunger type direct flow measuring devices, the DP switch nevertheless presents some difficulties of its own. One such problem rates relates to the hookup configuration typically employed with DP switches.
In particular, the high pressure connection of the DP switch is typically connected upstream of the coolant heat exchanger, and the low pressure connection of the DP switch is connected downstream of the coolant heat exchanger. In this way, the DP switch is able to sense the pressure drop across the coolant heat exchanger. However, because the coolant pressure at both the inlet and outlet of the coolant heat exchanger typically varies during system operations, the measured pressure differential across the coolant heat exchanger will likewise fluctuate, and may accordingly cause inaccurate coolant flow indications.
Another difficulty associated with the use of DP switches to facilitate coolant flow rate indications relates to the relatively small pressure drop typically experienced in the context of the x-ray tube cooling system heat exchangers. In particular, the typical DP switch is not sufficiently sensitive to be activated by less than 0.5 pounds per square inch differential pressure (xe2x80x9cPSIDxe2x80x9d). On the other hand, those DP switches which are sufficiently sensitive to respond to pressure differentials less than 0.5 PSID are, typically, relatively more expensive and physically larger than the more commonly used DP switches. Such an increase in cost is undesirable, and, the larger physical configuration precludes the use of such DP switches in many applications.
Another problem inherent in the use of DP switches concerns the number of fluid connections required to connect the DP switch to the cooling system. In particular, because the DP switch, by definition, must sense coolant pressure at two different points in the cooling system, a total of four fluid connections are required to establish fluid communication between the DP switch and the cooling system. In particular, the high pressure side of the DP switch must be connected to tubing which, in turn, is connected to the coolant system. The low pressure side of the DP switch must be connected in like fashion. As noted earlier, the introduction of such fluid connections in the cooling system increases the chances for system leaks and increases the overall maintenance burden associated with the cooling system.
At least one other configuration commonly employed to determine coolant flow rate in the x-ray tube cooling system involves the use of a pressure switch located at the cooling pump discharge line. Such pressure switches are distinct from DP switches in that the pressure switch is configured with a diaphragm or similar structure which is exposed on one side to atmospheric pressure, by way of a vent or the like in the switch body. The other side of the diaphragm in the pressure switch is exposed to system line pressure. These pressure switches thus measure the magnitude of the system line pressure in terms of pounds per square inch gage (PSIG). Because pressure switches are configured to measure pressure in terms of PSIG, they are relatively simple in construction and low in cost, as compared with DP switches which, as noted earlier, require two fluid connections to measure a pressure differential in PSID.
In operation, the coolant pump transfers energy to the coolant. If the cooling system is closed at some point such that the coolant is unable to flow, the energy thus transferred to the coolant is manifested primarily in the form of increased pressure in the coolant. If the cooling system is configured to permit flow of the coolant, the energy transferred to the coolant manifests itself in the form of pressure and velocity. Because the pressure switch is in fluid communication with the pump discharge line, the coolant leaving the pump acts on the diaphragm of the pressure switch and causes the pressure switch to generate a signal indicating that a particular pressure has been achieved in the pump discharge line. The pressure switch thus serves to verify that the cooling system pump in on line and transferring energy to the coolant.
While such configurations are effective in establishing the fact of increased coolant pressure, they are inadequate to provide meaningful feedback as to whether coolant is actually flowing. In particular, a situation could arise where a cooling system hose supplying the cooling system heat exchanger was kinked or obstructed in such a manner that no coolant flow was reaching the heat exchanger. However, the pressure switch located at the pump discharge would indicate that the coolant system was functioning because it would sense the pressure generated by the cooling pump. Thus, although no coolant would be reaching the heat exchanger in this example, the pressure switch would provide no indication whatsoever that there was a coolant system fault. Such a shortcoming is at best inconvenient and at worst may contribute to the failure of the x-ray device.
In view of the foregoing problems and shortcomings of existing x-ray tube cooling systems, it would be an advancement in the art to provide a cooling system configured to facilitate ready and reliable verification of coolant flow rates without compromising the overall operation of the cooling system or x-ray device. Further, the cooling system should be configured to facilitate implementation of corrective action in the event of a cooling system fault. Finally, the x-ray tube cooling system should be configured to minimize cost and reduce maintenance.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs which have not been fully or adequately solved by currently available x-ray tube cooling system. Thus, it is an overall objective of embodiments of the present invention to provide an x-ray tube cooling system which includes provisions for facilitating the monitoring of coolant flow parameters so as to enhance the overall operation and reliability of the x-ray device.
A related objective is to provide an x-ray tube cooling system which uses one or more coolant flow parameters to at least indirectly control the operation of the x-ray device.
In summary, these and other objects, advantages, and features are achieved with an improved cooling system for use in effecting heat transfer from an x-ray tube and for at least indirectly controlling the operation thereof. Embodiments of the present invention are well suited for use in conjunction with rotating anode or stationary anode x-ray tube configurations.
In one embodiment of the present invention, the cooling system includes a reservoir holding a volume of coolant in which at least apportion of the x-ray device is partially immersed. Preferably, the reservoir includes a flexible bladder, or the like, which serves to accommodate increases in coolant volume due to heat absorption. Because of the flexible nature of the bladder and the fact that one side of the bladder is exposed to the atmosphere, the coolant in the reservoir remains at atmospheric pressure. An outlet connection of the reservoir is joined to a fluid conduit which is in fluid communication with an external cooling unit. Another fluid conduit facilitates fluid communication between the external cooling unit and a pressure drop device of the x-ray device. Upstream of the inlet to the pressure drop device, a pressure tap is situated so as to be in fluid communication with the coolant flow. In one embodiment, the pressure tap is connected to the conduit joining the external cooling unit with the pressure drop device. A pressure switch is attached to the pressure tap so as be in simultaneous contact with the coolant in the conduit and with the coolant in the reservoir.
In operation, the external cooling unit generates a flow of coolant that is directed through the fluid conduit connecting the external cooling unit with the pressure drop device. As the fluid passes through the conduit, it also fills the pressure tap and comes into operative communication with the pressure switch attached to the pressure tap. In this way, the pressure switch is able to sense the pressure of the coolant in the conduit. Further, because the pressure switch is in communication with coolant contained in the reservoir, the pressure switch is also able to sense coolant pressure in the reservoir, and thus, the pressure differential.
Preferably, the pressure drop between the fluid conduit connecting the external cooling unit to the pressure drop device, and the reservoir, is facilitated by the pressure drop device. In particular, as the coolant from the external cooling unit passes through the pressure drop device and into the reservoir, the pressure drop device, by virtue of its physical configuration, induces a drop in pressure in the coolant passing therethrough.
Since the pressure switch is immersed in the coolant in the reservoir, the pressure switch has a relatively constant natural reference pressure with which to determine the aforementioned pressure differential. As a result of such arrangement, the pressure switch generally is not exposed to fluctuating pressure differentials which could compromise the accuracy of the results obtained by the pressure switch. Further, because the pressure switch is located on a pressure tap off the coolant supply line to the pressure drop device, and is not xe2x80x9cin-line,xe2x80x9d the pressure switch is able to sense the pressure differential in the cooling system without compromising the performance of the coolant system.
If the pressure differential sensed by the pressure switch is of a magnitude equal to or greater than the set point of the pressure switch, a circuit is completed, indicating that the rate of coolant flow has reached at least the minimum acceptable level. Preferably, the pressure switch is configured to close the circuit, thus indicating sufficient coolant flow, on rising pressure, and is configured to open the circuit, indicating insufficient coolant flow, on falling pressure.
In one embodiment, the pressure switch communicates with a controller, or the like, which is in communication with the x-ray device, so that in the event the differential pressure falls below an accepted range or value, the pressure switch can be used, at least indirectly, to shut down the x-ray device so as to prevent damage to the x-ray device from overheating as a result of inadequate coolant flow. Preferably, the pressure switch is also in electrical communication with a visual indicator, or the like, so that adequate coolant flow can be visually confirmed by the operator.
After exiting the pressure drop device, the coolant then enters the reservoir. In one embodiment, coolant entering the reservoir from the pressure drop device comes into contact with various structures of the x-ray device so as to absorb additional heat from the x-ray device. Ultimately, the coolant passing into the reservoir exits the reservoir by way of the exit connection and returns to the external cooling unit to repeat the cycle.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.